Cellulose acetate, particularly cellulose triacetate, which has excellent physical properties, particularly good processability and high optical properties, has been utilized for many years in the field of plastics, fibers, films (e.g., photographic film, etc.). Further, cellulose acetate has attracted attention from the viewpoint of the global environment because it possesses biodegradability.
In general, cellulose acetate is a semi-synthetic polymer obtained by esterification from cellulose as a starting material using acetic anhydride. Presently, commercially available cellulose acetates may be roughly divided into two groups according to the degree of acetylation thereof. One is cellulose triacetate (hereinafter referred to as CTA) having a degree of acetylation of not less than 59%. The other is cellulose diacetate of a wide range. Among them, those having a degree of acetylation of about 50 to 59% are referred to as cellulose diacetate (CDA). In other words, it is cellulose acetate soluble in acetone.
As for the use of cellulose acetate, in particular, CTA included, it can be used as various films including base materials for photographic film as well as fibers and medical materials. In general, a molded product of cellulose acetate is rigid and brittle. Such properties may become more significant the higher the acetylation degree becomes. Physical properties of polymeric materials greatly depend on their crystallizability. That is, those having a high crystallizability are imparted with strength while the flexibility thereof, for example, elongation is reduced, resulting in a brittle product. Of course, CTA is not an exception, and has high crystallizability due to the homogeneity of its structure. That is, in cellulose acetate, a higher degree of acetylation results in a higher crystallizability. In general, lower molecular weight material becomes a nucleus, forming a crystal. Accordingly, when CTA or CDA are used, a plasticizer is generally added to impart flexibility to the molded product. For example, acetate plastics used for a grip of a screw-driver and the like often utilizes a phthalate plasticizer such as diethyl phthalate. Cellulose acetate, particularly, CTA, is used as a raw material for various films due to its excellent transparency, while it has defects, for example, of providing a rigid and brittle film. To overcome its physical defects, a plasticizer is also used in this case. Addition of a component such as a plasticizer is accompanied not only with a poor yield of the final product due to bleedout during molding but also an economical disadvantage. Thus, cellulose acetate having excellent physical properties as well as characteristics of CTA has been expected.
On the other hand, due to the increase of the use of cellulose acetate, speeding up of processing techniques are required, and high speed molding, high speed spinning, high speed processing of the molded product have been tried. For example, in the production of a film, it is proposed to cast a solution of cellulose acetate at a high speed to mold into film. In order to improve the moldability corresponding to such a speeding up, it is proposed that the average degree of polymerization of cellulose acetate is reduced to the lower solution viscosity of cellulose acetate. However, when using cellulose acetate with a low degree of polymerization, the physical strength of the molded product is impaired.
Among cellulose acetates, cellulose triacetate, which has been used in various purposes, is crystalline and its solubility in a solvent is reduced as its crystallization properties increase. To improve the solubility in a solvent and moldability of cellulose triacetate, it is considered to be useful to make the cellulose triacetate amorphous or have a lower crystallizability by reducing the degree of substitution of the cellulose triacetate. However, when the degree of substitution of cellulose triacetate is reduced, the hygroscopicity is increased and the dimensional accuracy of the molded product is impaired.
Therefore, it is difficult to improve its solubility in a solvent, moisture resistance, moldability and the like by reducing the crystallizability of the cellulose acetate.
Accordingly, an object of the present invention is to further improve the characteristics of CTA and improve the physical properties of the molded product, and to provide a process for the production of such a cellulose acetate.
Another object of the present invention is to provide a cellulose acetate having low crystallizability and excellent moldability.
Still another object of the present invention is to provide a cellulose acetate having a high solubility in a solvent and moldability, in spite of having a high average degree of polymerization and high average degree of substitution.